1. Technical Field
The present invention relates to propylene block copolymers excellent in rigidity and impact resistance, and, more particularly, to propylene block copolymers produced by a continuous polymerization method, excellent in rigidity and impact resistance, and also in a balance of these two properties.
2. Background Art
As methods for improving the impact resistance of crystalline polypropylene, there have been known (1) a method in which propylene is copolymerized with a small amount of an -olefin; (2) a method in which polypropylene is mechanically blended with a rubber component; and (3) a so-called block copolymerization method in which polymerization for polymerizing propylene as a main component is firstly effected, followed by copolymerization of propylene with an .alpha.-olefin other than propylene.
However, the method (1) seems to have such a shortcoming that, although it brings about good effect of improving impact resistance, it provides greatly reduced rigidity. Further, the method (2) provides an improved balance of rigidity and impact resistance, but is economically disadvantageous because rubber that is used for blending is generally expensive.
The block copolymerization method (3) is most commonly used, in the light of such a background, to improve the impact resistance of crystalline polypropylene. The block copolymerization method is such that a rubber-like component which is a copolymer of propylene and ethylene or the like is produced in the presence of a polymer containing propylene as a main component. In this method, the two components are produced via successive polymerization processes, and then blended. Therefore, the block copolymerization method is also called chemical blend, and conveniently used.
For conducting block copolymerization, a continuous polymerization method in which respective blocks are continuously produced in separate polymerization tanks is more economically advantageous than a batch polymerization method in which respective blocks are produced in one polymerization tank. In such a continuous polymerization method, it is common to polymerize propylene as a main component in the first stage, and to carry out polymerization for producing a rubber-like component in the second stage. In particular, the most commonly used is a method in which propylene homopolymer is produced in the first stage, and a rubber-like component that is an ethylene-propylene copolymer usually called EPR is produced in the second stage. It is noted that the first stage and second stage are not always correspond to polymerization tanks; and there is also a case where a plurality of polymerization tanks are used for each stage.
In general, propylene block copolymers obtained by continuous polymerization methods tend to be poor in impact resistance as compared with those ones obtained by batch polymerization methods.
In order to solve this problem, methods utilizing classification have been proposed, for example, in Japanese Patent Laid-Open Publications No. 135987/1976 and No. 116716/1980. Further, there has also been known such a method in which a certain compound is added to a polymerization tank that is used for producing a rubber-like component in the second stage. For instance, Japanese Patent Laid-Open Publication No. 115417/1980 discloses the addition of tertiary amines, ketones, ethers, esters, acid amides, phosphoric amides and the like; Japanese Patent Laid-Open Publication No. 147508/1982 discloses the addition of halogen-containing aluminum compounds; Japanese Patent Laid-Open Publication No. 69821/1986 discloses the addition of alcohols; Japanese Patent Laid-Open Publication No. 116618/1987 discloses the addition of glycol ethers; and Japanese Patent Laid-Open Publication No. 152116/1989 discloses the addition of hydrogen sulfide.
However, it cannot be said that these methods are sufficiently effective for improving the balance of rigidity and impact resistance. An object of the present invention is therefore to provide, by a continuous polymerization method, propylene block copolymers which have physical properties comparable to those of propylene block copolymers obtained by batch polymerization methods.